In an electro-photographic image forming apparatus, such as an image copier, printer, facsimile and a compound apparatus having the functions of the above apparatus, firstly, latent images of a document are formed on a photosensitive drum, after toner is applied to the latent images to become actual, next the actual toner images are transferred onto a recording material, after which the toner images on the recording material are fixed, whereupon the recording material carrying the fixed images is ejected from the electro-photographic image forming apparatus.
Further, in the case of color image formation, latent images of Y (yellow), M (magenta), C (cyan) and K (black), which correspond to colors of the document, are formed on four separate photosensitive drums, the actual four color toner images are firstly transferred onto an intermediate transfer belt, and which are secondarily transferred onto the recording material, then the toner images are fixed to be ejected from the color image forming apparatus.
Such fixing device to fix the toner images on the recording material, incorporates a fixing roller which houses a halogen lamp as a heating means, and a pressure applying roller to press against the fixing roller, whereby a nipping section, which is structured of the fixing roller and the pressure applying roller, nips the recording material to be conveyed, and also heats it to fix the image, which is a fixing device as a heat roller fixing method. The structure of this type of fixing device is very simple, which is commonly used in various image forming apparatuses.
In this fixing device, in order to speed up the fixing operation, a large amount of heat energy is required for the toner and the recording material, therefore, the nipping width is required to be increased. For this purpose, it is also conceivable to increase the force, with which the pressure applying roller presses against the fixing roller, the thickness of the elastic layer of the pressure applying roller, which is formed of silicon rubber, and the diameter of each of the above two rollers.
However, if the force of the pressure applying roller and the thickness of the elastic member are increased, the nipping width can become uneven with respect to the axis of the roller, which results in uneven fixing and generation of creases of the recording material. Further, if the diameter of the rollers is increased, the overall size of the fixing device becomes larger, as well requiring a longer warm-up time.
In order to overcome these problems, a fixing device is disclosed in Unexamined Japanese Patent Application Publication No. 2004-109,878, which incorporates a rotating fixing roller which has an elastic layer formed of silicon rubber and houses a halogen lamp as a heating means, an endless fixing belt which is driven by the fixing roller, and a compressible pad which is mounted against the inner peripheral surface of the fixing belt, so that the compressible pad presses the fixing belt against the fixing roller.
According to this fixing device, since the fixing belt is pressed against the fixing roller by a compressible pad, the fixing belt deforms elastically whereby a wide nipping section is generated between the fixing roller and the fixing belt. Accordingly, acceleration of fixing is realized without any size increase of the fixing device. Further, the heat capacity of the fixing belt is so small that the warm-up time is reduced, as well as energy saving is performed.
In the fixing device of Unexamined Japanese Patent Application Publication No. 2004-109,878, since the inner peripheral surface of the looped and rotating fixing belt comes into contact with the compressible pad which does not rotate, the sliding(rubbing) resistance between them is quite large, which results in various problems. That is, poor quality of fixed images due to slippage of the fixing belt, power consumption of a motor which drives the fixing belt increases due to increased driving torque of the fixing belt, reduction gears are stressed and may be damaged, and the inner peripheral surface of the fixing belt is abraded away. To reduce the sliding resistance between the inner peripheral surface of the fixing belt and the compressible pad, a special lubricant applying member is mounted to be pressed against the inner peripheral surface of the fixing belt, and which is formed of a porous material, such as a sponge to apply silicon oil as a lubricant. Further, a low sliding sheet is mounted between the compressible pad and the fixing belt.
However, the lubricant, existing between the fixing belt and the sliding member, penetrates into the axial direction due to capillary action, and further, it turns to appear on the surface of the fixing belt, so that the fixing belt slips against the fixing roller, and eventually the lubricant attaches itself to the recording material, which result in poor image formation.
Accordingly, in Unexamined Japanese Patent Application Publication No. 2004-109,878, a countering member is mounted at both sides of the sliding member to prevent any leakage of the lubricant.
This however, increases the size of the fixing device in the axial direction of the fixing device, as well as increasing production cost.
An object of the present invention is to provide a fixing device in which the lubricant between the fixing belt and the sliding member is prevented from penetrating into the axial direction caused by capillary action and from appearing on the surface of the fixing belt, without using any specific leakage preventing member, and further to provide an image forming apparatus incorporating the same fixing device.